ARRL

Secure Site Login

Forum Home - Rules - Help - Login - Forgot Password
Members can access, post and reply to the forums below. Before you do, please first read the RULES.

LC Pass Filters

Mar 2nd 2016, 22:50

K7LMC

Joined: Mar 20th 2012, 17:26
Total Topics: 0
Total Posts: 0
Hi, all. I'm working on a couple of homebrew antenna projects that require splitting the signals from 2 antennae to a single dual band radio.

1 one for LEO work on the 2m/70cm and the other for specialty Search and Rescue application on 2m/1.25m

When constructing the LC filter circuit, the value of the caps and inductors are always said to be of equal value. However, when using a 5 pole configuration for chebysev filters (which is best for the 2m/1.25m filter due to the bands being so close) the values of components are not equal.

And then, finding caps that match those values is impossible. So, my question is do I simply need to get close to the sum value of the component based on it being in series or parallel? e.g. I need to match the total capacitance of the circuit.

Or does the circuit need a specific capacitance at the specific location in the circuit?

Thanks for the help.
Mar 2nd 2016, 23:22

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
You generally do need a specific capacitance in specific location. But, there are clever tricks that engineers use to get standard values--you may be able to tweak the amount of passband ripple or exact bandwidth/cutoff frequencies. The most straightforward approach may be to use two capacitors in parallel to get an oddball capacitance value..

Zack Lau W1VT
ARRL Senior Lab Engineer
Mar 2nd 2016, 23:35

K7LMC

Joined: Mar 20th 2012, 17:26
Total Topics: 0
Total Posts: 0
Thanks for the feedback, Zack. One followup question... when using caps in parallel, will that work for the caps to ground, also?

~Leonard, K7LMC
Mar 2nd 2016, 23:50

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Yes, you can use them for capacitors to ground. It can get tricky at VHF/UHF due to parasitics--conventional leaded capacitors may have too much inductance--but these days you can use tiny chip capacitors. Advanced Technical Ceramics makes very good microwave chip capacitors. When I was in school a fellow in a research lab mentioned that they bought their capacitors just so they could obtain the silver strap leads.

Computer modeling or measurement is the only practical way to see how much stray inductance/capacitance is too much.

Zack Lau W1VT
ARRL Senior Lab Engineer
Mar 3rd 2016, 00:19

K7LMC

Joined: Mar 20th 2012, 17:26
Total Topics: 0
Total Posts: 0
That's great. Thanks so much for the information. One final question. Does the same principal apply for using the caps in series also. For example if C1 in the circuit calls for 43.23pf parallel to ground, could I use 1 68pf and 1 120pf in series for a capacitance of 43.4pf?

I think, that's my last question, lol. Appreciate the help.

~Leonard, K7LMC
Mar 3rd 2016, 01:45

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Yes, you can use capacitors in series as well.

Zack Lau W1VT
ARRL Senior Lab Engineer

Back to Top